This invention is generally related to direct current (D.C.) motors, and more particularly to brushless D.C. motors which are capable of operating at extremely high speeds while maintaining a relatively low operating temperature.
The advantages that brushless D.C. motors have over conventional D.C. motors are well known. In brushless D.C. motors, it is usually most practical to provide a stator structure where the windings are placed in an external, slotted stator. The rotor consists of a shaft and a hub assembly with a magnetic structure. Such brushless D.C. motors produce their output torque via the interaction between a magnetic field produced by the permanent magnet rotor, and a magnetic field due to a D.C. current in the stator structure.
A conventional D.C. motor, on the other hand, is usually composed of permanent magnets which are situated in an outer stator structure and an inner rotor carrying the various winding coils. When compared with the typical brushless D.C. motor, it becomes evident that there are significant differences in winding and magnet locations. The conventional D.C. motor has the active conductors in the slots in the rotor structure, and in contrast, the brushless D.C. motor has the active conductors in slots in the outside stator. The removal of heat produced in the active windings is, thus, easier to accomplish in a brushless D.C. motor since the thermal path to the environment is shorter. Furthermore, since the permanent magnet rotor of a brushless motor does not contribute any heating in and of itself, the brushless D.C. motor is a more stable mechanical device than its conventional D.C. motor counterpart from a thermal point of view.
Brushless D.C. motors, just as conventional D.C. motors, are used to produce mechanical power from electric power. As such, both may be viewed as energy converters. Neither the brushless D.C. motor nor its conventional counterpart, however, are ideal converters due to various motor losses. Motor losses can generally be classified into two categories: (1) load sensitive losses which are dependent upon the generated torque; and (2) speed sensitive losses which are proportional to the motor's rotational speed.
The load or torque sensitive losses are generally limited to winding losses which are proportional to the square of the current going through the windings times the resistance of the windings. Motors are often wound with copper conductors, or in some of the newer low-inertia type motors, aluminum conductors. With either metal, however, electrical resistance increases with temperature, each at a different rate. Therefore, for a given value of current the winding losses will increase as a motor heats up.
Speed sensitive losses, such as core or iron losses due to eddy currents and hysteresis, windage and friction, short circuit currents, and brush contact, when combined together act as a velocity dependent torque which opposes the output torque of the motor. In brushless D.C. motors, brush contact losses are obviated as are friction losses between the brushes and commutator of a given conventional D.C. motor. Iron or core losses due to eddy currents and the hysteresis effect, as well as short circuit currents, remain as dominant losses even in brushless D.C. motors. As is well known, eddy currents are phenomena caused by a change of magnetic field through a medium that can also support a flow of electric current. In the case of a conventional permanent magnetic motor, the medium that experiences the change of magnetic field in which a potential voltage is induced is the iron of the armature. Likewise, the housing portion of a brushless D.C. motor also has a potential voltage induced in ts and produces currents called eddy currents. In either case, the induced eddy currents which are produced in the iron are proportional to speed, and can have a significant heating effect on the motor particularly when it operates at high speed.
Short circuit currents also contribute a component of loss which increases with motor speed. Motors which are not otherwise limited by their iron losses have speed limitations due to short circuit currents. Eddy currents, short circuit currents and hysteresis determine the maximum speed that may be obtained from conventional D.C. motors as well as brushless D.C. motors.
One prior art approach is disclosed in U.S. Pat. No. 4,130,769 Karube. Karube discloses a brushless D.C. motor having a rotor composed of a permanent field magnet, an armature coil body containing a plurality of polygonally-shaped coils with a given number of turns, and Hall effect sensors positioned in proximity to the end face of the magnet. The arrangement of the armature coil body and cylindrical permanent magnet, however, promotes two basic problems which would lead to heat caused by short circuit currents. First, the structure of the magnet yields an ill-defined magnetic field between adjacent poles. Separate and distinct magnets with alternating poles and a highly directional magnetic field would be more preferable to induce a sharp definition between the poles, and thus, promote a more distinct switching effect during commutation and minimize cogging. Second, the positioning of Hall effect sensors or any other commutation sensing means would more preferably be in the direction of the magnetic field, not as in Karube, perpendicular from the field where only flux leakage would be detected.
A second basic trend in the design of D.C. low inertia motors utilizes the moving coil concept. This principle basically consists of a multiple d'Arsonval movement with a commutation arrangement. Moving coil structures which are presently used have followed two general design paths, both of which have a multitude of conductors which move in a magnetic field, the armature structure being supported mainly by non-magnetic materials and the active conductors therefore moving in an air gap with a high magnetic flux density. Since the moving coil motor does not have moving iron in its magnetic field, neither iron eddy currents nor hysteresis effects are predominant as heat producing motor losses. Consequently, typical moving coil motors require lower power inputs to obtain high rotational speeds. One major problem with such moving coil or low-inertia motors, however, is that their armature-to-housing thermal resistance and housing-to-ambient thermal resistance are greatly different. For example, typical moving coil motors have thermal time constants of ranging from about 500 milliseconds and to about one second for armature-to-housing, and 30 to 60 minutes for housing-to-ambient. It is readily apparent that the armature of such moving coil motors could be heated to destructive temperatures in less than a minute without the motor's giving any warning because of its long thermal time constant between housing and ambient.
In order to prevent thermal destruction, therefore, air cooling is often provided for moving coil motors. Other forms of heat dissipation, such as cooling the motor with circulating water or oil, have also been employed. As is evident, the use of a moving coil low inertia motor which requires peripheral equipment to cool it would be cumbersome and more costly, especially in applications such as hand-held surgical tools which necessitate light weights and long-term heat dissipation capabilities. It would, therefore, be desirable to provide a brushless D.C. motor offering high efficiency and good commutation, while at the same time being capable of operating at high rotational speeds for extended periods of time without necessitating the use of forced air or other cooling techniques.